[M-IS16] 津波堆積物

[MIS16-P06] 地質調査に基づく八丈島の古津波履歴と規模の検討

*海田 比呂子¹、後藤 和久¹、石澤 尭史²、林 薫³、井村 春生¹ (1.東京大学、2.東北大学、3.八丈町教育委員会)

キーワード：津波堆積物、古津波、八丈島

After the 2011 off the Pacific coast of Tohoku Earthquake and tsunami, it became a priority to consider various earthquake and tsunami risks by considering historical and geological records of past earthquakes and tsunamis (The Headquarters for Earthquake Research Promotion, 2011). However, there are only limited number of studies on small islands especially along the Izu-Bonin Trench. Considering this situation, we conducted a field survey in Hachijo Island. Four large trenches (T1 ~ T4) were excavated at Yaene, Okago, near the site where Sugihara & Shimada (1998) had reported tsunami deposits. There was a thick event layer covering a crack in the main outcrop of T1 (elevation 14.8 m). The layer was identified as tsunami deposit by the components and sedimentary structure. The result of radiocarbon dating using a piece of charcoal collected in the soil sample beneath the tsunami deposit in T1 was 1484-1644 cal AD (2σ range). Thus, it is likely that this tsunami deposit could have been formed by the historical tsunami during the Edo period: either the Keicho (1605.2.3) or the Genroku (1703.12.31) tsunamis. As for the outcrop of T4, which was excavated under a 40-ton sized boulder, there was an event layer consisted of well-rounded cobbles. The layer together with a boulder was potentially deposited by the past tsunami event but further careful research is required. Although precise earthquakes and tsunamis that caused these traces is unclear, this study suggests that one of the few tsunamis that hit Hachijo Island in the Edo period was even larger than previously considered based on the historical documents. Our results suggest the need of re-evaluating source models of past earthquakes and tsunamis by using geological evidence in Hachijo Island.